The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Turbojet engines generate significant noise pollution. There is a strong demand to reduce the noise emitted by turbojet engines, particularly as the latter become increasingly powerful.
It is known that a significant portion of this noise is created in the air intake zone of the nacelle inside which the engine is housed. In that zone, there is an engine fan, which creates very strong suction and shearing noise from the air.
It is known in particular to form the walls of the nacelle with sound attenuation panels, so as to reduce the noise created in that zone.
These panels are typically made up of one or more cellular core layers (called “honeycomb” structure) whereof the outer face, i.e., the face radially furthest from the axis of the engine, is covered with a skin that is impermeable to the air, and the inner face of which, i.e., the face thereof radially closest to the axis of the engine, is covered with an air-permeable skin.
These sound attenuation panels may further comprise a multi-perforated skin, called a septum, between the different cellular core layers.
Each panel is assembled by positioning the different skins and cellular core layers glued on a mold having the required shape.
The assembly undergoes curing in a furnace so as to tighten the layers and polymerize the pieces.
Such panels form acoustic resonators capable of “trapping” the noise, and therefore attenuating the sound emissions toward the outside of the nacelle.
For reasons for example related to the geometry of the nacelle or structural constraints, it is often necessary to place several cellular core blocks end to end to form the sound attenuation panels.
More particularly, in reference to FIG. 1, the sound attenuation panels 1 are often formed by several cellular core structures 2, 3 placed end to end, with different thicknesses.
Examples for instance include an assembly of a first bilayer cellular core structure 2 and a second bilayer cellular core structure forming two blocks with different thicknesses intended to be joined in a junction area J, the assembly of the two blocks being covered by shared inner 4 and outer 5 skins.
These are then called distributed sound attenuation panels.
In this type of distributed sound attenuation panel, the different thickness of the two cellular core structures to be joined may be due to the size of the cells of the honeycomb layers and/or the number of cellular core layers of each structure to be joined.
In this type of panel, further, the acoustic properties, i.e., the noise absorption level as a function of the frequency and sound level of the noise, depend in particular on the joining of the cellular core structure(s).
In this case, the junction areas between the structures must be treated with very special care if one wishes to preserve good sound absorption and effectiveness of the distributed sound panels.
One common technique used to connect the cellular core structures to each other consists of coating the adjacent edges of the structures with a glue that turns into foam upon curing, and thereby forms a sort of connecting strip of expanded material between those edges.
The advantage of this technique is that it makes it possible to obtain a panel which, from a mechanical perspective, behaves as if it were made in a single block.
More particularly known is a method for manufacturing distributed sound panels in which, in reference to FIG. 2, a junction between two cellular core structures 20, 21 is provided by the presence of an intumescent adhesive 23.
However, the presence of such an intumescent element 23 generates a rupture in the acoustic treatment inasmuch as the cells of the honeycomb structures may be plugged by that adhesive 23, and the holes of the acoustic skins and septa, if applicable, may also be plugged by that adhesive 23.
Further known are methods without adhesive wherein, preferably at the edge-to-edge junction of the cellular core structures, the cells situated on the adjacent edges of the structures intended to be joined are opened, and those structures are joined edge-to-edge by fitting the open cells into one another.
Such a method does not guarantee good junctions in the context of sound attenuation panels formed on parts with a strong curvature, such as cylindrical nacelles.
It is then necessary to use outside indexing means to ensure the proper positioning of the different acoustic structures relative to one another, of the pencil on part type.
It also does not make it possible to prevent two septa of two respective adjacent cellular structures to be joined from overlapping in the junction area of the two structures.
This has the drawback of affecting the acoustic quality of the panel.